The present invention relates to a semiconductor device such as a field effect thin film transistor, and more particularly, it relates to a semiconductor device comprising a highly reliable and stable semiconductor layer with excellent operation characteristics, i.e., a layer of polycrystalline silicon thin film semiconductor, as the principal portion thereof.
With increasing size of scanning circuit portions of image scanners such as long-arrayed optical sensors and large-area two dimensional optical sensors for image reading, or of driver circuits of image display devices based on, for example, liquid crystal materials (referred to simply hereinafter as "LCs"), electrochromic materials (referred to simply hereinafter as "ECs"), or electroluminescent materials (referred to simply hereinafter as "ELs"), it is proposed to use the thin films of silicon deposited on a predetermined substrate as the base material for the circuits.
Thin films of silicon is a well known material, and they have been widely used in faster and in more functional large image readers and image display devices. To this end, polycrystalline silicon (polysilicon) films are preferred to amorphous ones. More specifically, polysilicon thin films yield far higher effective carrier mobility .mu..sub.eff as compared to amorphous silicon thin films obtained by an ordinary process of discharge decomposition which yields an .mu..sub.eff as low as 0.1 cm.sup.2 /V.multidot.sec. Higher effective carrier mobility .mu..sub.eff is essential for a material constituting the scanning circuits of a high speed and high performance image readers and or for a material constituting the driver circuits of high quality image display devices, and, moreover, the amorphous silicon thin films above suffers a severe deterioration with the passage of time. For example, the amorphous silicon thin film obtained by ordinary discharge decomposition suffers a decrease in drain current and a drift of the transistor threshold voltage with increasing duration of applied DC voltage to the gate. In contrast to the disadvantages of the amorphous silicon thin film, a polysilicon thin film yields a far higher effective carrier mobility .mu..sub.eff, and, theoretically, there is still a possibility of fabricating a thin film having a yet higher mobility .mu..sub.eff.
In practice, however, the devices based on the polycrystalline silicon thin films fabricated according to various processes obtained upto the present are still unsatisfactory in performance and reliability.
In the light of such circumstances, the present inventors have found that, in a semiconductor device based on polycrystalline silicon thin film, the performance and the reliability of the device depend on the crystallographic orientation of the silicon thin film.